Add onAppear and onDisappear support

Sources/OpenSwiftUI/Core/Update/Update.swift

@@ -71,9 +71,24 @@ enum Update {
 
     @inline(__always)
     static func dispatchActions() {
-        // FIXME
-        for action in actions {
-            action()
+        guard !actions.isEmpty else {
+            return
+        }
+
+        let actions = Update.actions
+        Update.actions = []
+        performOnMainThread {
+            // TODO: Signpost.postUpdateActions
+            begin()
+            for action in actions {
+                let oldDepth = depth
+                action()
+                let newDepth = depth
+                if newDepth != oldDepth {
+                    fatalError("Action caused unbalanced updates.")
+                }
+            }
+            end()
         }
     }
 
@@ -107,5 +122,5 @@ extension Update {
 // FIXME: migrate to use @_extern(c, "xx") in Swift 6
 extension MovableLock {
     @_silgen_name("_MovableLockSyncMain")
-    static func syncMain(lock: MovableLock ,body: @escaping () -> Void)
+    static func syncMain(lock: MovableLock, body: @escaping () -> Void)
 }

Sources/OpenSwiftUI/Core/View/View/ViewInputs.swift

@@ -116,6 +116,12 @@ public struct _ViewInputs {
         return newInputs
     }
 
+    // MARK: - base.phase
+    @inline(__always)
+    var phase: Attribute<_GraphInputs.Phase> {
+        base.phase
+    }
+
     // MARK: - base.changedDebugProperties
 
     @inline(__always)

Sources/OpenSwiftUI/View/Modifier/AppearanceActionModifier.swift

@@ -0,0 +1,159 @@
+//
+//  AppearanceActionModifier.swift
+//  OpenSwiftUI
+//
+//  Audited for RELEASE_2021
+//  Status: Blocked by _makeViewList
+//  ID: 8817D3B1C81ADA2B53E3500D727F785A
+
+// MARK: - AppearanceActionModifier
+
+internal import OpenGraphShims
+
+/// A modifier that triggers actions when its view appears and disappears.
+@frozen
+public struct _AppearanceActionModifier: PrimitiveViewModifier {
+    public var appear: (() -> Void)?
+    public var disappear: (() -> Void)?
+
+    @inlinable
+    public init(appear: (() -> Void)? = nil, disappear: (() -> Void)? = nil) {
+        self.appear = appear
+        self.disappear = disappear
+    }
+
+    public static func _makeView(
+        modifier: _GraphValue<Self>,
+        inputs: _ViewInputs,
+        body: @escaping (_Graph, _ViewInputs) -> _ViewOutputs
+    ) -> _ViewOutputs {
+        let effect = AppearanceEffect(modifier: modifier.value, phase: inputs.phase)
+        let attribute = Attribute(effect)
+        attribute.flags = [.active, .removable]
+        return body(_Graph(), inputs)
+    }
+
+    public static func _makeViewList(
+        modifier: _GraphValue<Self>,
+        inputs: _ViewListInputs,
+        body: @escaping (_Graph, _ViewListInputs) -> _ViewListOutputs
+    ) -> _ViewListOutputs {
+        fatalError("TODO")
+    }
+}
+
+// MARK: - AppearanceEffect
+
+private struct AppearanceEffect {
+    @Attribute
+    var modifier: _AppearanceActionModifier
+    @Attribute
+    var phase: _GraphInputs.Phase
+    var lastValue: _AppearanceActionModifier?
+    var isVisible: Bool = false
+    var resetSeed: UInt32 = 0
+    var node: AnyOptionalAttribute = AnyOptionalAttribute()
+
+    mutating func appeared() {
+        guard !isVisible else { return }
+        defer { isVisible = true }
+        guard let lastValue,
+              let appear = lastValue.appear
+        else { return }
+        Update.enqueueAction(appear)
+    }
+
+    mutating func disappeared() {
+        guard isVisible else { return }
+        defer { isVisible = false }
+        guard let lastValue,
+              let disappear = lastValue.disappear
+        else { return }
+        Update.enqueueAction(disappear)
+    }
+}
+
+// MARK: AppearanceEffect + StatefulRule
+
+extension AppearanceEffect: StatefulRule {
+    typealias Value = Void
+
+    mutating func updateValue() {
+        #if canImport(Darwin)
+        if node.attribute == nil {
+            node.attribute = .current
+        }
+
+        if phase.seed != resetSeed {
+            resetSeed = phase.seed
+            disappeared()
+        }
+        lastValue = modifier
+        appeared()
+        #else
+        fatalError("See #39")
+        #endif
+    }
+}
+
+#if canImport(Darwin) // See #39
+
+// MARK: AppearanceEffect + RemovableAttribute
+
+extension AppearanceEffect: RemovableAttribute {
+    static func willRemove(attribute: OGAttribute) {
+        let appearancePointer = UnsafeMutableRawPointer(mutating: attribute.info.body)
+            .assumingMemoryBound(to: AppearanceEffect.self)
+        guard appearancePointer.pointee.lastValue != nil else {
+            return
+        }
+        appearancePointer.pointee.disappeared()
+    }
+
+    static func didReinsert(attribute: OGAttribute) {
+        let appearancePointer = UnsafeMutableRawPointer(mutating: attribute.info.body)
+            .assumingMemoryBound(to: AppearanceEffect.self)
+        guard let nodeAttribute = appearancePointer.pointee.node.attribute else {
+            return
+        }
+        nodeAttribute.invalidateValue()
+        nodeAttribute.graph.graphHost().graphInvalidation(from: nil)
+    }
+}
+#endif
+
+// MARK: - View Extension
+
+extension View {
+    /// Adds an action to perform before this view appears.
+    ///
+    /// The exact moment that OpenSwiftUI calls this method
+    /// depends on the specific view type that you apply it to, but
+    /// the `action` closure completes before the first
+    /// rendered frame appears.
+    ///
+    /// - Parameter action: The action to perform. If `action` is `nil`, the
+    ///   call has no effect.
+    ///
+    /// - Returns: A view that triggers `action` before it appears.
+    @inlinable
+    public func onAppear(perform action: (() -> Void)? = nil) -> some View {
+        modifier(_AppearanceActionModifier(appear: action, disappear: nil))
+    }
+
+    /// Adds an action to perform after this view disappears.
+    ///
+    /// The exact moment that OpenSwiftUI calls this method
+    /// depends on the specific view type that you apply it to, but
+    /// the `action` closure doesn't execute until the view
+    /// disappears from the interface.
+    ///
+    /// - Parameter action: The action to perform. If `action` is `nil`, the
+    ///   call has no effect.
+    ///
+    /// - Returns: A view that triggers `action` after it disappears.
+    @inlinable
+    public func onDisappear(perform action: (() -> Void)? = nil) -> some View {
+        modifier(_AppearanceActionModifier(appear: nil, disappear: action))
+    }
+}

Tests/OpenSwiftUICompatibilityTests/View/Modifier/AppearanceActionModifierTests.swift

@@ -0,0 +1,33 @@
+//
+//  AppearanceActionModifierTests.swift
+//  OpenSwiftUICompatibilityTests
+
+import Testing
+
+#if canImport(Darwin)
+struct AppearanceActionModifierTests {
+    @Test
+    func appear() async throws {
+        struct ContentView: View {
+            var confirmation: Confirmation
+
+            var body: some View {
+                AnyView(EmptyView())
+                    .onAppear {
+                        confirmation()
+                    }
+            }
+        }
+
+        #if os(iOS)
+        await confirmation { @MainActor confirmation in
+            let vc = UIHostingController(rootView: ContentView(confirmation: confirmation))
+            vc.triggerLayout()
+            workaroundIssue87(vc)
+        }
+        #endif
+    }
+
+    // TODO: Add disappear support and test case
+}
+#endif